The field of this invention relates generally to an antenna and more particularly to an antenna which is designed primarily to have particular utility when used with FM radio receivers.
An antenna is a device for transmitting or receiving radio waves. A transmitting antenna converts the electrical signals from a transmitter (radio, television or radar) into an electromagnetic wave, which spreads out from the transmitter. A receiving antenna intercepts this wave and converts it back into electrical signals, then amplified and decoded by a receiver, such as a radio, television or radar set.
A radio transmitter produces a signal in the form of an alternating electric current, that is one which oscillates rapidly back and forth along a wire. The rate of this oscillation can be anything from thousands of times a second to billions of times per second. This rate of oscillation is known as a frequency and is either measured in kilohertz (thousands of times a second) or, for higher frequencies, in megahertz (millions of times a second).
The oscillating current in the transmitting antenna produces an electromagnetic wave around it which spreads out from the antenna like the ripples in a pond. This wave sets up electric and magnetic fields. The lines of the electric field run along the antenna and those of the magnetic field around the antenna. Both the electric and magnetic fields oscillate in time with the electric current.
Wherever this wave comes into contact with a receiving antenna, it induces a small electric current in it which alternates back and forth along the receiving antenna in time with the oscillations of the wave. Although this current is much weaker than one in the transmitting antenna, it can be picked up by the amplifier of the radio to receive it.
The air is full of radio waves at all frequencies which the antenna picks up indiscriminately. Each receiver has a means of selecting a narrow band of frequencies at any time. This is what happens when a particular signal is tuned in. Each set can be tuned within a certain frequency range and will only respond to signals in that range. For example, common FM frequencies range from 88 to 108 mHz. It would be desirable to design the receiving antenna to be sensitive only to these frequencies.
Antennas are subject to resonance. What is meant by resonance is that the antenna may be relatively insensitive to much of the frequency range (for example 88 to 108 megahertz [mHz]), but between 95 and 97 mHz the antenna is exceedingly sensitive. Another antenna may be constructed to be sensitive between 89 and 91 while a third could be constructed to be sensitive between 102 and 104 mHz. Therefore, for a particular FM receiver to be sensitive to the full range of desired frequencies it would be necessary to utilize a plurality of different antennas in conjunction with a single receiver.
At 98 mHz, the electromagnetic wave produced is 120 inches in length. It will greatly increase the efficiency of the antenna if the length of the antenna is directly related to the wavelength of the signal it receives. However, it is just not feasible to construct antennas 120 inches in length. Therefore, it is common to utilize an antenna which has a length exactly one-half or one-quarter of the wave length it receives. However, even a thirty inch or sixty inch antenna is rather difficult to be utilized in conjunction with a receiver and, if it is desired to have that receiver to be sensitive to all frequencies within its receiving band, it will be necessary to employ a plurality of such antennas.
Another common requirement utilized in conjunction with antennas is directivity. If the transmitting antenna is positioned vertically, the receiving antenna must be positioned vertically. If the transmitting antenna is set horizontally, the receiving antenna must be set horizontally. In other words for the best results, the receiving antenna should be set at exactly the same angle as the transmitting antenna.
In order to diminish the physical size of an antenna, it has been common to utilize an electrically conducted loop. Obviously, a thirty inch loop will assume a size of less than nine and one-half inches in diameter. Using such loops, antenna size can be substantially diminished. For best reception, the plane of the loop should pass through the transmitting signal. Therefore, the performance of a receiver with a loop antenna is directly dependent on the positioning of the antenna.
There is a need to construct an antenna which is sensitive to a wide bandwidth, small in size and which is less sensitive to unwanted radio waves.